学步园

用户名表管理地区表，显示二级双地区信息</p> <p>select f.add_id,(select a1.name from my.law_area a1 where a1.id = a.parent_id) as ,a.name from my.law_firm f left join my.law_area a on f.area_id = a.id</p> <p>select * from my.law_firm f left join my.law_area a on f.area_id = a.id left join my.law_area a1 on a.parent_id = a1.id</p> <p>select 嵌套、关联查询优化</p> <p>1. select .... from (subquery) as name ... ;</p> <p>实例：<br /> select a1,a2,a3<br /> from (select b1 as a1,b2 as a2,b3 as a3 from bbb) as aaa</p> <p> where a1 > 1;</p> <p>2.用exists替代in嵌套查询：</p> <p>实例：</p> <p> select a1 from aaa</p> <p> where a2 in(select b2 from bbb where b3 >10 ); 效率很低</p> <p> select a1 from aaa</p> <p> where exists(select b2 from bbb where bbb.b1=aaa.a1);效率最高</p> <p> 3.去掉字段中有'北京'的部分</p> <p>update edb_bookweb_using.edb_hotelinfo</p> <p>set hot_name_cn= trim(replace(hot_name_cn,'北京',''))</p> <p>where hot_name_cn like '%北京%';<br />